Internet DRAFT - draft-vesco-vcauthtls
draft-vesco-vcauthtls
WG A. Vesco
Internet-Draft L. Perugini
Intended status: Standards Track LINKS Foundation
Expires: 19 August 2024 16 February 2024
Transport Layer Security (TLS) Authentication with Verifiable Credential
(VC)
draft-vesco-vcauthtls-01
Abstract
This document defines a new certificate type and extension for the
exchange of Verifiable Credentials in the handshake of the Transport
Layer Security (TLS) protocol. The new certificate type is intended
to add the Verifiable Credentials as a new means of authentication.
The resulting authentication process leverages a distributed ledger
as the root of trust of the TLS endpoints' public keys. The
endpoints can use different distributed ledger technologies to store
their public keys and to perform the TLS handshake.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-vesco-vcauthtls/.
Source for this draft and an issue tracker can be found at
https://github.com/Cybersecurity-LINKS/draft-vesco-vcauthtls.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 August 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
3. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. client_certificate_type and server_certificate_type
extensions . . . . . . . . . . . . . . . . . . . . . . . 5
4. did_methods extension . . . . . . . . . . . . . . . . . . . . 6
5. TLS Client and Server Handshake . . . . . . . . . . . . . . . 7
5.1. ClientHello message . . . . . . . . . . . . . . . . . . . 8
5.2. ServerHello message . . . . . . . . . . . . . . . . . . . 9
5.3. CertificateRequest message . . . . . . . . . . . . . . . 10
5.4. Certificate message . . . . . . . . . . . . . . . . . . . 10
5.5. CertificateVerify message . . . . . . . . . . . . . . . . 10
6. TLS handshake Examples . . . . . . . . . . . . . . . . . . . 10
6.1. Server authentication with Verifiable Credential . . . . 10
6.2. Mutual authentication with Verifiable Credentials . . . . 11
6.3. Mutual authentication with Client using Verifiable
Credential and Server using X.509 Certificate . . . . . . 12
6.4. Mutual authentication with Client using X.509 Certificate
and Server using Verifiable Credential . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
The Self-Sovereign Identity (SSI) is a decentralised identity model
that gives an entity control over the data it uses to generate and
prove its identity. SSI model relies on three fundamental elements:
a distributed ledger as the Root of Trust (RoT) for public keys,
Decentralized IDentifier [DID], and Verifiable Credential [VC]. An
SSI aware entity builds his identity starting from generating its key
pair (_sk_, _pk_). Then the entity stores _pk_ in the distributed
ledger of choice for other entities to authenticate it. An entity's
DID is a pointer to the distributed ledger where other entities can
retrieve its _pk_. A DID is a Uniform Resource Identifier (URI) in
the form did:did-method-name:method-specific-id where method-name is
the name of the [DID] Method used to interact with the distributed
ledger and method-specific-id is the pointer to the [DID] Document
that contains _pk_, stored in the distributed ledger. After that,
the entity can request a VC from one of the Issuers available in the
system. The VC contains the metadata to describe properties of the
credential, the DID and the claims about the identity of the entity
and the signature of the Issuer. The combination of the key pair
(_sk_, _pk_), the DID and at least one VC forms the identity
compliant with the SSI model. An entity requests access to services
by presenting a Verifiable Presentation [VP]. The VP is an envelop
of the VC signed by the entity holding the VC with its _sk_. The
verifier authenticates the entity checking the validity and
authenticity of the VP and the inner VC before granting or denying
access to the requesting entity. Figure 1 shows step by step the
generation of the identity and the authentication with VP.
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--------
| Entity |
| |
--------
identity = [{pk,sk}]
--------
| Entity | pk -----
| | -----------------------------------------------> | DLT |
-------- | |
identity = [{pk,sk},DID] -----
-------- request VC --------
| Issuer | <---------------- | Entity |
| | ----------------> | |
-------- VC --------
identity = [{pk,sk},DID,VC]
-------- VP(VC) ---------- DID resolve -----
| Entity | ----------------> | Verifier | ----------------> | DLT |
| | <---------------- | | <---------------- | |
-------- ok/ko ---------- pk -----
Figure 1: Generation of the identity compliant with the SSI model and
authentication with VP
The current implementations of the authentication process run at the
application layer. A client estabhlishes a TLS channel
authenticating the server with the server's X.509 certificate. Then
the server authenticates the client that sends its VP at application
layer (i.e. over the TLS channel already established). The mutual
authentication with VPs occurs when also the server exchanges its VP
with the client again at application layer.
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SSI is emerging as an identity option for Internet of Thing and Edge
devices in computing continuum environments. In these scenarios,
(mutual) authentication with VP can take place directly at the TLS
protocol layer, enabling the peer-to-peer interaction model envisaged
by the SSI model. This document describes the extensions to TLS
handshake protocol to support the use of VCs for authentication while
preserving the interoperability with TLS endpoints that use X.509
certificates. The extensions enable server and mutual authentication
using VC, X.509, Raw Public Key or a combination of two of them. The
ability to perform hybrid authenticated handshakes supports the
gradual deployment of SSI in existing systems. Moreover, the
extension allows TLS endpoints to use different distributed ledger
technologies to store their public keys and to authenticate the
peers. The authentication process is successful if the TLS endpoints
implement the DID Method to resolve the peer's DID.
This document uses _italic formatting_ in the following sections to
mark some paragraphs discussing items still under design: Section 5.2
and Section 5.4.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Extensions
3.1. client_certificate_type and server_certificate_type extensions
The TLS extensions client_certificate_type and
server_certificate_type defined in [RFC7250] are used to negotiate
the type of Certificate messages used in TLS to authenticate the
server and, optionally, the client. This section defines a new
certificate type, called VC, for the TLS 1.3 handshake. The updated
CertificateType enumeration, the corresponding addition to the
CertificateEntry structure, and the Certificate message structure are
shown below. CertificateType values are sent in the
server_certificate_type and client_certificate_type extensions, and
the CertificateEntry structures are included in the certificate chain
sent in the Certificate message.
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/* Managed by IANA */
enum {
X509(0),
RawPublicKey(2),
VC(TBD),
(255)
} CertificateType;
struct {
select(certificate_type){
// The new certificate type defined in this document
case VC:
opaque cert_data<1..2^24-1>;
// RawPublicKey certificate type defined in RFC 7250
case RawPublicKey:
opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
// X.509 certificate defined in RFC 5246
case X509:
opaque cert_data<1..2^24-1>;
};
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
As per [RFC7250], the client will send a list of certificate types in
[endpoint]_certificate_type extension(s), the server processes the
received extension(s) and selects one of the offered certificate
types, returning the negotiated value in the EncryptedExtensions
message. Note that there is no requirement for the negotiated value
to be the same in client_certificate_type and server_certificate_type
extensions sent in the same message. Client and server can use
different certificate types as long as the peer is able to verify
that specific type of certificate.
4. did_methods extension
This section defines the did_methods extension, used as part of an
extended TLS 1.3 handshake when VC certificate type is used.
ExtensionType now contains the did_methods entry.
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enum {
server_name(0),
max_fragment_length(1),
..,
did_methods(TBD),
(65535)
} ExtensionType;
This extension contains a list of DID Methods an endpoint supports,
i.e. a set of DLTs an endpoint can interact with to resolve the
peer's DID. A client MUST send this extension in the extended
ClientHello message only when it indicates Verifiable Credential
support in the server_certificate_type extension. The server MUST
send this extension in a CertificateRequest message only if it
indicates Verifiable Credential in client_certificate_type extension.
The extension format which uses the extension_data field, is used to
carry the DIDMethodList structure. The structure of this new
extension is shown below.
enum {
btcr(0),
ethr(1),
iota(2),
..
(65535)
} DIDMethod
struct {
DIDMethod did_methods<2..2^16-2>
} DIDMethodList
The list of existing DID Methods is currently maintained by the W3C
in [DID-Registries]. Each DID Method is expressed in the form of a
string. This document proposes the DIDMethod enum to map these
strings into integer values.
5. TLS Client and Server Handshake
Figure 2 shows the message flow for full TLS handshake.
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DLT Client Server DLT
Key ^ ClientHello
Exch | + server_certificate_type*
| + client_certificate_type*
| + did_methods*
| + signature_algorithms*
v + key_share* -------->
ServerHello ^ Key
+ key_share* v Exch,
{EncryptedExtensions} ^ Server
{+ server_certificate_type*} | Params
{+ client_certificate_type*} |
{CertificateRequest*} |
{+ did_methods*} v
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
DID Resolve
<==========
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
DID Resolve
==========>
[Application Data] <---> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
{} Indicates messages protected using keys
derived from a
[sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 2: Message Flow for full TLS Handshake
5.1. ClientHello message
To express support for VC certificate type, a client MUST include the
extension of type client_certificate_type or server_certificate_type
in the extended ClientHello message as described in Section 4.1.2 of
[RFC8446]. If the client sends the server_certificate_type extension
indicating VC, it MUST also send the did_methods extension.
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5.2. ServerHello message
When the server receives the ClientHello message containing the
server_certificate_type extension and/or the client_certificate_type
extension, the following scenarios are possible:
* The server does not support the extensions, omits them in
EncryptedExtensions and the handshake proceeds with X.509
certificate(s).
* The server does not support any of the proposed certificate types
and terminates the session with a fatal alert of type
unsupported_certificate.
* Both client and server indicate support for the VC certificate
type. The server selects VC certificate type, but the client did
not send the did_methods extension in addition to the
server_certificate_type extension. The server MUST terminate the
session with a fatal alert of type missing_extension.
* Both client and server indicate support for the VC certificate
type. The server selects VC certificate type, but the server's
DID is not compatible with any of the DID Methods supported by the
client and listed in the did_methods extension sent with the
ClientHello message. _This document defines two possible server
behaviours (a) the server terminates the session with a fatal
alert of type unsupported_did_methods, (b) the server sends a
HelloRetryRequest (HRR) message with a new extension listing the
DLTs in which it owns a DID_. _These design considerations apply:
solution (a) requires defining a new fatal alert message type, and
the client has no clues to perform a new successful TLS handshake;
solution (b) requires defining a new HRR extension which could
have privacy implications as it discloses the DLTs where the
server owns its DIDs; on the other hand, this extension provides
the client with clues to retry a successful new TLS handshake_.
* Both client and server indicate support for the VC certificate
type, the server MAY select the first (most preferred) certificate
type from the client's list that is supported by both endpoints.
It MAY include the client_certificate_type in the
EncryptedExtensions message to request a certificate from the
client. In case the server selects VC certificate type, it MUST
also send the did_methods extension in the CertificateRequest
message.
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5.3. CertificateRequest message
The server sends the CertificateRequest message to request client
authentication. It MUST include the did_methods extension if it
indicates VC in the client_certificate_type extension. If the
ClientHello contains the did_methods extension, the server MUST send
a list of DID Methods client and server have in common. If the
client does not send the did_methods extension the server MUST select
a list of DID Methods it supports. A client that processes the
CertificateRequest message that does not own a DID compatible with
the DID Methods selected by the server MUST send a Certificate
message containing no certificates, i.e. with the certificate_list
field having length 0.
5.4. Certificate message
When the selected certificate type is VC, the certificate_list in the
Certificate message MUST contain no more than one CertificateEntry
with the content of the endpoint's Verifiable Credential. _This
document intends to mandate CBOR encoding for the Verifiable
Credential_. After decoding, the endpoint MUST follows the procedure
in [VC] to verify the Verifiable Credential.
5.5. CertificateVerify message
As discussed in Section 1, an Holder wraps its own Verifiable
Credential into a Verifiable Presentation and signs it before
presenting it to a Verifier for authentication purposes. During the
TLS handshake, when the selected certificate type is VC, the
subsequent CertificateVerify message acts also as the Holder
signature on the Verifiable Presentation. In fact, the signature is
computed over the transcript hash that contains also the Verifiable
Credential of the sender inside the Certificate message.
6. TLS handshake Examples
This section shows some examples of TLS handshakes using different
combinations of certificate types.
6.1. Server authentication with Verifiable Credential
The example in Figure 3 shows a TLS 1.3 handshake with server
authentication. The client sends the server_certificate_type
extension indicating both VC and X.509 certificate types. In
addition, the client sends the did_methods extension with the list of
supported DID Methods. The client does not own an identity at the
TLS level, therefore omits the client_certificate_type extension.
The server selects VC certificate type, sends the EncryptedExtensions
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message with the server_certificate_type extension set to VC, and
sends its Verifiable Credential into the Certificate message. After
receiving the CertificateVerify and Finished messages, the client
resolves the server's DID to retrieve the server _pk_ and
authenticate it.
DLT Client Server
ClientHello
server_certificate_type=(VC,X.509)
did_methods=(btcr,iota) -------->
ServerHello
{EncryptedExtensions}
{server_certificate_type=VC}
{Certificate}
{CertificateVerify}
{Finished}
<-------- [Application Data]
DID Resolve
<==========
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 3: TLS Server Uses Verifiable Credential
6.2. Mutual authentication with Verifiable Credentials
The example in Figure 4 shows a TLS 1.3 handshake with mutual
authentication where both client and server authenticate the peer
using Verifiable Credentials. The client sends the
server_certificate_type extension indicating both VC and X.509
certificate types along with the did_methods extension containing the
list of supported DID Methods. The client also sends the
client_certificate_type extension indicating its capability to
provide both a Verifiable Credential and an X.509 certificate. The
server sends the server_certificate_type set to VC, the
client_certificate_type set to VC and the CertificateRequest message
with the did_methods extension containig a set of DID Methods in
common with the client. Client and server send their Verifiable
Credential into their respective Certificate messages. After
receiving the CertificateVerify and Finished messages, the client and
then the server resolve the peer's DID to retrieve the associated
_pk_ and authenticate each other.
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DLT Client Server DLT
ClientHello
server_certificate_type=(VC,X.509)
client_certificate_type=(VC,X.509)
did_methods=(btcr,ethr)
-------->
ServerHello
{EncryptedExtensions}
{server_certificate_type=VC}
{client_certificate_type=VC}
{CertificateRequest}
{did_methods=(btcr,ethr)}
{Certificate}
{CertificateVerify}
{Finished}
<-------- [Application Data]
DID Resolve
<==========
{Certificate}
{CertificateVerify}
{Finished} -------->
DID Resolve
==========>
[Application Data] <-------> [Application Data]
Figure 4: TLS Client and TLS Server Use Verifiable Credentials
6.3. Mutual authentication with Client using Verifiable Credential and
Server using X.509 Certificate
The example in Figure 5 shows a TLS 1.3 handshake with mutual
authentication that combines the use of Verifiable Credential and
X.509 certificate. The client uses a Verifiable Credential, and the
server uses an X.509 certificate. The client sends the
server_certificate_type extension indicating X.509 certificate types.
The client also sends the client_certificate_type extension
indicating its capability to provide both a Verifiable Credential and
an X.509 certificate. The server sends the server_certificate_type
set to X.509, the client_certificate_type set to VC and the
CertificateRequest message with the did_methods extension containig
the set of suported DID Methods. The server sends its X.509
certificate and the client its Verifiable Credential into their
respective Certificate messages. After receiving the
CertificateVerify and Finished messages, the server resolves the
client DID to retrieve the client _pk_ and authenticate it.
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Client Server DLT
ClientHello
server_certificate_type=(X.509)
client_certificate_type=(VC,X.509)
-------->
ServerHello
{EncryptedExtensions}
{server_certificate_type=X.509}
{client_certificate_type=VC}
{CertificateRequest}
{did_methods=(btcr,ethr,iota)}
{Certificate}
{CertificateVerify}
{Finished}
<-------- [Application Data]
{Certificate}
{CertificateVerify}
{Finished} -------->
DID Resolve
==========>
[Application Data] <-------> [Application Data]
Figure 5: TLS Client Uses a Verifiable Credential and TLS Server
Uses an X.509 Certificate
6.4. Mutual authentication with Client using X.509 Certificate and
Server using Verifiable Credential
The example in Figure 6 complements the previous one showing a TLS
1.3 handshake with mutual authentication where the client uses X.509
certificate and the server a Verifiable Credential. The client sends
the server_certificate_type extension indicating both VC and X.509
certificate types along with the did_methods extension containing the
list of supported DID Methods. The client also sends the
client_certificate_type extension indicating its capability to
provide only an X.509 certificate. The server sends the
server_certificate_type set to VC, the client_certificate_type set to
X.509 and the CertificateRequest message. The server sends its
Verifiable Credential, and the client its X.509 certificate into
their respective Certificate messages. After receiving the
CertificateVerify and Finished messages, the client resolves the
server's DID to retrieve the server _pk_ and authenticate the client.
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DLT Client Server
ClientHello
server_certificate_type=(VC,X.509)
client_certificate_type=(X.509)
did_methods=(btcr,ethr,iota)
-------->
ServerHello
{EncryptedExtensions}
{server_certificate_type=VC}
{client_certificate_type=X.509}
{CertificateRequest}
{Certificate}
{CertificateVerify}
{Finished}
<-------- [Application Data]
DID Resolve
<==========
{Certificate}
{CertificateVerify}
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 6: TLS Client Uses an X.509 Certificate and TLS Server
Uses a Verifiable Credential
7. Security Considerations
All the security considerations presented in [RFC8446] applies to
this document as well. Further considerations can be made on the DID
resolution process. Assuming that a DID resolution is performed in
clear, a man-in-the-middle could impersonate the DLT node, forge a
DID Document containing the authenticating endpoint's DID, associate
it with a key pair that he owns, and then return it to the DID
resolver. Thus, the attacker is able to compute a valid
CertificateVerify message by possessing the long term private key.
In practice, the man-in-the-middle attacker breaks in transit the
immutability feature provided by the DLT, i.e. the RoT for the public
keys. A possible solution to this attack is to esthablish a TLS
channel towards the DLT node and authenticate only the latter to rely
on the received data. The DLT node MUST be authenticated through an
X.509 certificate. The session resumption and 0 round-trip time
(0-RTT) features of TLS 1.3 can be used to reduce the overhead of
establishing this TLS channel. In addition, the communication with
the DLT node can be protected with Internet Protocol Security (IPsec)
[RFC6071] and Internet Key Exchange (IKE) [RFC5996] in endpoint-to-
endpoint transport mode for even better performance in term of
latency of DID resolution.
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8. IANA Considerations
To be addressed
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, DOI 10.17487/RFC5996, September 2010,
<https://www.rfc-editor.org/rfc/rfc5996>.
[RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
DOI 10.17487/RFC6071, February 2011,
<https://www.rfc-editor.org/rfc/rfc6071>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/rfc/rfc7250>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
9.2. Informative References
[DID] W3C, "Decentralized Identifiers (DIDs) v1.0", July 2022,
<https://www.w3.org/TR/did-core/>.
[DID-Registries]
W3C, "DID Specification Registries", September 2023,
<https://www.w3.org/TR/did-spec-registries/#did-methods>.
[VC] W3C, "Verifiable Credentials Data Model v2.0", November
2023, <https://www.w3.org/TR/vc-data-model-2.0/>.
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[VP] W3C, "Verifiable Credentials Data Model v2.0", November
2023, <https://www.w3.org/TR/vc-data-model-2.0/>.
Acknowledgments
We would like to thank Nicola Tuveri for his very helpful suggestions
during the preparation of the first version of this technical
specification.
Authors' Addresses
Andrea Vesco
LINKS Foundation
Email: andrea.vesco@linksfoundation.com
Leonardo Perugini
LINKS Foundation
Email: leonardo.perugini@linksfoundation.com
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